Microwave noise generator



June 18, 1968 A.j|`.WACHTENHE1M ET AL. 3,389,347

MI CROWAVE NOISE G ENERATOR Filed Sept. 8, 1966 INVENTORS,

ARTHUR I. WACHTENHEIM BRUNO W. ZOTTER BY ,m

L ATTORNEYS United States Patent O 3,389,347 MICROWAVE NOISE GENERATOR Arthur I. Waehtenheim, Lakewood, and Bruno W. Zotter, Belmar, NJ., assignors to the United States of America as represented by the Secretary of the Army Filed Sept. 8, 1966, Ser. No. 578,439 3 Claims. (Cl. S31-J8) ABSTRACT OF THE DISCLOSURE An electron beam type microwave noise generator wherein cyclotron and synchronous waves or modes are generated and are propagated on a generated axial electron beam. A suitable coupler is adapted to `couple either to the cyclotron waves or synchronous waves. For cyclotron coupled waves, the axial magnetic field in the vicinity of the interaction region is made relatively high and the axial magnetic field in the vicinity of the electron beam generating means is made relatively low. For synchronous coupled waves, the axial magnetic eld in the vicinity of the interaction region is made relatively low while the axial field in the vicinity of the electron beam generating means is made relatively high. The amplified noise is coupled out from the coupling means after interaction with the beam.

This invention relates to microwave noise generators and more particularly to an electron beam wave type noise generator.

In carrying out direct noise measurements for high power TWT type amplifiers, it is usually required to provide a noise source with an equivalent noise temperature comparable or .larger than the noise temperature of the amplifier under test. Noise sources presently available in the microwave region usually comprise gas discharge tubes having an equivalent temperature of under 20,000" K. At best, such gas discharge tubes only provide accurate noise measurements to about 20 db. Noise figures much larger than 20 db, corresponding to an equivalent noise temperature of 30,000 K., can not be accurately measured with such gas discharge tubes as the noise source. For noise figures greater than 20 db a measurement of noise power output is required and the gain must be known to calculate the noise figure. Optimization under conditions where, not only the noise figure varies, but also the gain varies, is very tedious and time consuming.

It is therefore an object of the present invention to provide an improved noise source adapted for use in the microwave region wherein the above noted limitations are overcome.

The present invention is based on the wel-l known principle that cyclotron Waves or modes propagate on a beam moving in the Z (axial direction) with constant axial velocity vo wherein there is a finite axial magnetic focussing field. The A-C motion of the electron beam is not confined to the axial direction; there can be transverse A-C motion. In the case of transverse motion of the electrons in a finite eld, it is Well known that the electrons will execute a circular motion at the cyclotron frequency. This type of mode is known as the cyclotron mode and may be characterized as either fast or slow cyclotron modes or waves. These cyclotron modes are transverse velocity modes. Also, under the above specified conditions, two other modes or waves propagate on the beam and are known as synchronous waves. These are characterized as either positive or negative and are essentially transverse displacement modes. The cyclotron and synchronous waves are simultaneously excited bythe thermal velocities of the electrons leaving the cathode and are simultaneously propagated on the electron beam. The

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amplitudes of the cyclotron waves are proportional to the temperature of the cathode. However, if the magnetic field along the beam is different from the magnetic field at the cathode, the amplitude is multiplied by the ratio of these magnetic fields.

In accordance with the present invention there is provided an electron beam type microwave noise generator which includes means for generating an axial electron beam having cyclotron and synchronous waves or modes propagated thereon. Also included are means interacting with said beam and in coupling relationship to one of said modes. Included further are means for extracting said coupled mode as noise energy, and a collector electrode for intercepting said electron beam. For the cyclotron waves, a relatively high axial magnetic field is provided inthe vicinity of the coupling means and a relatively low axial magnetic field is provided in the vicinity of the electron generating means. For the synchronous waves, a relatively high axial magnetic field is provided in the vicinity of the electron generating means and a relatively low axial magnetic field is provided in the vicinity of the coupling means. In both cases, the noise-` of the beam generating means is multiplied at the output of the coupling means by a factor determined by the ratio of the two axial magnetic fields.

The invention will be more clearly understood by considering the non-limitative embodiment thereof illustrated in the single figure of the accompanying drawing.

Referring now to the drawing, there is shown a noise generator tube 10 having a Pierce lgun 12 at one end thereof for providing a converged axial beam of electrons moving with a constant velocity vo and which is lintercepted by a conventional collector anode 14 at the other end of the tube. Coupled to the beam and coaxially aligned therewith intermediate the Pierce gun 12 and collector 14 is a Cuccia coupler 16. The Cuccia coupler 16 is well known in the art and may be considered :as a simple parallel plate coupler in which the linearly polarized transverse field is independent of the axial coordinate Z. A relatively large axial magnetic field B1 is provided in the vicinity of the Cuccia coupler 16, hereinafter referred to as the interaction region, by means of a solenoid 18 which may be energized by any suitable means (not shown). As shown, the magnetic field B1 extends from the electron beam minimum as it leaves the cathode of gun 12 to beyond the terminus of Cuccia coupler 16. The magnetic field B1 is chosen such that its cyclotron frequency we equals the signal frequency f=2.8 gHz. per kilogauss. A relatively low axial magnetic field B2 is provided in the vicinity of the Pierce gun 12, but opposite to the direction B1, by means of solenoid 20 which may be energized by any suitable means. For purposes of clarity, the relative D-C potentials applied between the Pierce gun 12 and collector 14 to provide the constant beam velocity vo have been omitted from the drawing.

In order to separate the gun region 12 from the magnetic field B1 in the interaction region, a pair of focussing pole pieces are provided as shown at 21 intermediate Cuccia coupler 16 and the gun 12. Due to the bucking or opposition of the magnetic fields B1 and B2, a very low magnetic field is produced in the vicinity of the cathode of gun 12 and the residual field (B1-B2) may be represented by the arrow B3. Hence, the ratio of the magnitudes of the magnetic fields B1 at coupler 16 and B3 at the cathode of gun 12 can be made as high as 1000 to 1 or higher. That is, B1 may have a value of 1000 gauss while that of B3 may have a value of 1 gauss. As is well known, the Cuccia coupler 16 will couple only to a wave having a zero propagation factor, i.e., to the fast cyclotron wave. Assuming that wc is the frequency of the fast cyclotron wave being coupled to Cuccia coupler 16 and occ is the wcc 3 With the temperature at the cathode of Pierce gun l2 l being in the order of 1000 K. to 2000 K., it can be seen that for cyclotron waves, the noise temperature is multiplied by 1000, or the ratio of cyclotron frequency coupled by the Cuccia coupler 16 to the cyclotron frequency at the cathode of Pierce gun 12?. The amplified microwave noise energy may be coupled out of Cuccia coupler 16 by any suitable coupling means as shown at 22.

Although the invention has been described in connection with a Cuccia coupler and the fast cyclotron wave, it is to be understood that any other coupler tnatyvill couple to either one of the cyclotron waves may also be used. It is also to be understood that a synchronous wave and synchronous coupler may also be used to provide noise generation. In this case however, the magnetic field at the cathode of -gun 12 must be made larger than the magnetic field in the vicinity of the synchronous coupler.

While there has been described what is at present considered to be the preferred embodiment of this invention, it will be obvious to those skilled in the art that various changes and modifications may be made therein without departing from the invention, and it is therefore aimed in the appended claims to cover all such changes and modifications as fall within the true spirit and scope of the invention.

What is claimed is:

1. An electron beam type microwave noise generator comprising, means for generating an axial electron beam adapted to propagate cyclotron and synchronous Waves, a collector electrode for intercepting said beam, means intermediate said electron generating means and said collector electrode interacting with said beam and in coupling relationship to either of the cyclotron waves on said beam, respective discrete means for producing a relatively high axial magnetic eld in the vicinity of said coupling means and a relatively low axial magnetic field in the vicinity of said electron beam generating means, and means for extracting noise energy from said coupling means.

2. The noise generator in accordance with claim 1 wherein said coupling means comprises a Cuccia coupler.

3. An electron beam type microwave noise generator comprising, means for generating an axial electron beam adapted to propagate cyclotron and synchronous waves, a collector electrode for intercepting said beam, means intermediate said electron generating means and said collector electrode interacting with said beam and in coupling relationship to either of the `synchronous waves on said beam, respective discrete means for producing a relatively low axial magnetic eld in the vicinity of said synchronous wave coupling means and a relatively high axial magnetic eld in the vicinity of said electron beam generating means, and means for extracting noise energy from said coupling means.

References Cited UNITED STATES PATENTS ROY LAKE, Primary Examiner.

S. H. GRIMM, Assistant Examiner. 

